This disclosure relates generally to the field of fluids used in subsurface wellbore, or pipeline cleaning. More specifically, the disclosure relates to the use of amphiphilic blends and friction reducers to enhance removal of sand and cuttings from wellbores, particularly after multi-zone fracturing operations.
Wellbores drilled through subsurface formations include vertical and “horizontal” wells, i.e., those that are highly inclined from vertical up to and exceeding 90 degrees from vertical. The objective of drilling such wells is to penetrate a large area of a subsurface formation or formations that exist in layers having approximately the same orientation with respect to vertical, that is, the formations are highly inclined with respect to vertical.
Some such formations include naturally or mechanically fractured zones, that is, zones where the formation has enhanced hydraulic permeability and conductivity due to tectonic stresses. Some fractured zones contain hydrocarbons in the form of oil and/or gas. A horizontal wellbore with fractured formation enhances the hydraulic interconnection of a large surface area of the formation to a single conduit, thereby making it economically feasible to produce the hydrocarbons.
It is also known in the art to stimulate such wellbores through such formations, as well as non-horizontal wellbores, after completion of the wellbore by hydraulic fracturing. Hydraulic fracturing may further enable hydraulic connection of the formation located at a substantial lateral distance from the wellbore, thus enhancing the productivity of the wellbore further. Hydraulic fracturing includes pumping a liquid suspension of “proppant”, which is a granular material whose grains are suitable to hold open a fracture created by the pressure of pumping the suspension into the formation after the pumping pressure is relieved.
Hydraulic fracturing may be performed in some wellbores in a plurality of individual zones isolated by seal elements disposed in the wellbore after the individual zone has been hydraulically fractured. It may be necessary to remove the mechanical obstructions, excess cuttings and other debris from the wellbore in order to gain access to the entire horizontal wellbore for fracturing. It may also be necessary to remove excess fracture proppant, other debris and to remove the seal elements from the wellbore after hydraulic fracturing operations are finished in order to produce hydrocarbons commercially from the wellbore. The removal of debris, seals, and excess proppant from horizontal wellbores is typically facilitated through the use of a coil-tubing rig or workover rig equipped with a bottom-hole tool assembly designed for milling debris into small particles and/or flushing the obstructions up the wellbore annulus to the surface.
It is known in the art that fluid system additives can be used to enhance well-bore cleaning operations. Generally, friction reducers such as high molecular weight polymers of acrylates, acrylamides, and derivatives thereof can be added to the fluid system to reduce pressure pumping requirements and build viscosity in the fluid. Furthermore, sand and debris removal from horizontal wellbores may be performed with intermittently pumped high viscosity “pills” of gel disposed in a high viscosity, concentrated water solution. Typically, the gel is a xanthan, guar, polyacrylamide (or derivative thereof), cellulostic polymer (or derivative thereof), or other polymeric material, which may be the same as used to suspend the proppant in the hydraulic fracturing operation. It is believed that using the gel pill as described performs the function of a “liquid squeegee”, that is, to sweep the sand and other debris out of the wellbore by the function of the high-viscosity of the gel pill. It is further believed that high viscosity solutions are necessary to keep solid particles/debris in suspension as they travel through the annulus so as to prevent sedimentation and accumulation of solids within the wellbore behind the bottom-hole tool assembly, thereby reducing the probably of the debris causing the tubing and tools to become stuck in the wellbore.
Although the existing art provides means to enable horizontal well completions operations, the frequency of operational failures due to solids accumulation and production-related failures to do the debris left in the annulus after clean-out procedures creates the need for sand and debris removal fluids having higher solids removal efficiency than those known in the art.